1. Field of Invention
This invention relates to an electrostatically actuated microwave or millimeter wave MEMS switch and switch array. More specifically, it relates to an electrostatically actuated microwave or millimeter wave switch with at least one resistive line to achieve the switch actuation and to achieve the DC to RF isolation simultaneously. It also relates to an electrostatically actuated microwave or millimeter wave MEMS switch with a resistive de-actuation electrode to de-actuate the switch.
2. Brief Description of the Prior Arts
A switch is the basic building block for many microwave and millimeter wave (hereinafter called microwave or RF for simplicity) circuits and units in order to achieve switching and routing of microwaves, generation of phase shift of the microwaves. Examples of applications include microwave switching, routing, phase shifting, beam forming and phase array antennas. Conventional microwave switches are made from semiconductors such as Si or GaAs into field effect transistor or P-I-N form. The semiconductor microwave switches operate based on the non-linear current versus voltage characteristics and have certain advantages including high switching speed, compactness and well developed technologies. However, due to the non-linear current-voltage characteristics and the need to maintain a specific operating point in the On-state, there are several drawbacks. The drawbacks of the semiconductor microwave switches include: [1] signal distortion, [2] relatively large insertion loss and [3] high DC power consumption in On-state.
In order to develop a new generation of microwave systems for communications and phased arrays, extensive work has been carried out on the development of miniature mechanical switches based on micro-electro-mechanical technology (herein after call MEMS technology). The MEMS technology for the fabrication of microwave switches can be categorized as those based on electromagnetic force actuation and those on electrostatical force actuation. Several authors have disclosed microwave MEMS switch structures based on electromagnetic actuation. Examples are U.S. Pat. No. 6,016,092 entitled “Miniature Electromagnetic Microwave Switches and Switch Arrays” and U.S. Pat. No. 6,310,526 B1 entitled “Double-throw Miniature Electromagnetic Microwave Switches” by the present inventors.
The efforts on electrostatically actuated microwave MEMS switches are even more extensive [H. J. De Los Santos, Yu-Hua Kao, A. L. Caigoy and E. D. Ditmars, “Microwave and Mechanical Considerations in the Design of MEM Switches for Aerospace Applications”, Proceedings of IEEE Aerospace Conference, Vol. 3, pp. 235-254, 1997]. The main advantage of the electrostatically actuated microwave MEMS switches is the avoidance of the electromagnetic coils for the actuation of the switches. FIGS. 1(a), 1(b) and 1(c) show schematic diagrams of a conventional electrostatically actuated microwave MEMS switch (1). On a dielectric substrate (2), an input transmission line (3) and an output transmission line (4) are created for the transmission of microwave signals. There is a gap (5) with a length of (Lgap, in FIG. 1(c)), separating the input transmission line (3) and the output transmission line (4). A movable contact pad (6) with a length of Lpad is suspended over and covers parts of the input and output transmission lines (3, 4). Two inner supports (8, 9), which are parts of a dielectric membrane (9′, in FIGS. 1(b) and 1(c)), are used to connect the contact pad (6) to two actuation top electrodes (10, 11). These two actuation top electrodes (10, 11) are further connected through two outer supports (12, 13, in FIG. 1(a)) to two anchors (14, 15). The two anchors (14, 15) are finally connected to the substrate (2). It is noted that the inner supports (8, 9) are electrically insulating so that the propagating microwave signals along the input transmission line (3) and the output transmission line (4) when the switch is actuated will not be affected by the prescribe of the actuation top electrodes (10, 11). On the actuation top electrodes (10, 11), the outer supports (12, 13) and the anchors (14, 15), there is a layer of metal (16,17) for connecting actuating voltage to the actuation top electrodes (10, 11). Under the actuation top electrodes (10, 11), two actuation bottom electrodes (18, 19) are deposited directly below the actuation top electrodes (10, 11). These actuation bottom electrodes (18, 19) are connected through conductors (20, 21) to bottom connecting pads (22, 23). When a DC voltage is applied between the actuation top electrodes (10, 11) and the actuation bottom electrodes (18, 19), electrostatic forces induced will actuate the movable contact pad (6) towards the overshadowing portions of the input transmission line (3) and the output transmission line (4) and causing a short circuit between the two. Microwave signals propagating from the input transmission line (3) now can reach the output transmission line (4). In the above-described structure, in order to reduce the interference of the propagating microwave signals due to the presence of the actuation top electrodes (10,11), the length (Ls) of the inner supports (8, 9) must be sufficiently large. Hence, the mechanical strength of the inner supports (8, 9) may not be sufficient to maintain the gap (24) between contact pad (6) and the input and output transmission lines (3, 4). Furthermore, after the actuation of the contact pad (6), there may be an attracting force between the contact pad (6) and the input and output transmission lines (3, 4). This will make de-actuating of switch (1) to be difficult when the DC power source is switched off.
In order to reduce the above-mentioned drawbacks, there is an alternate microwave switch structure (30) proposed as shown in FIGS. 2(a) and 2(b), where the connection between input transmission line (31) and output transmission line (32) on a substrate (33) is achieved by a movable contact pad (34), which is connected electrically to two anchors (35, 35′) through two supports (36, 36′). One of the anchors (35, 35′) is connected to a contact pad (37). Under the movable contact pad (34), there is an actuation bottom electrode (38), which is connected through a conductive connecting line (39) to a connecting pad (40). The advantage of this structure (30) is the improved mechanical properties. However, since the movable contact pad (34) and the actuation bottom electrode (38) are connected directly to the DC power supply through connecting pads (37, 40), there will be significant loss of the propagating microwave signals. In order to minimize the un-wanted microwave loss, RF chokes (41, 42) are added between the DC power supply and the movable contact pad (34), and between the DC power supply and the actuation bottom electrode (38). The required RF chokes (41, 42) are often bulky and still may lead to interference of the propagating microwaves.
From the above descriptions, it is evident that microwave units or switches can be simplified further if an electrostatically actuated microwave MEMS switch can be provided without the need of the RF chokes and yet to achieve effective isolation between the DC power source and the propagating RF signals. In this invention, an electrostatically actuated MEMS switch structure without the need of the RF chokes to achieve both the actuation and the effective isolation between the DC power source and the propagating microwave signals is provided.